The present invention relates to the fabrication of thermistors, or the like electrical components. The thermistors being fabricated are quite thin, i.e. wafers. A thermistor has a pair of electric contacts defined on one side of its body and the contacts are separated by a gap. Wafer thermistors can be initially fabricated in a large, flat sheet from which the thermistors are thereafter separated.
Small size, wafer-shaped thermistors are beneficial for obtaining a rapid, accurate response in a clinical thermometer, as shown in U.S. Pat. No. 4,317,367. Heretofore, fabrication of such wafer thermistors was not practical, as the electrical resistances of the thermistors had to be accurately rated following their fabrication, and this could only conveniently be done with relatively larger size thermistors.
Recently, techniques have been developed for fabricating thermistors of relatively small dimensions and which are quite thin in their thickness dimensions so that they are wafers. An inexpensive, rapid technique for accurately rating a small size wafer thermistor has also been developed, as shown in U.S. Pat. No. 4,200,970.
Wafer thermistors of quite small size, e.g. 0.060".times.0.060".times.0.010" are fabricated by initially forming a quite thin sheet of ceramic thermistor ware of considerably larger length and width dimensions, coating each of the opposite surfaces of the thermistor ware with a respective uniform, unbroken layer of electric contact material, e.g. a silver compound, as disclosed in U.S. Pat. No. 4,200,970, scoring the sheet for defining the individual thermistors to be broken from the sheet and also for defining a plurality, i.e. two, separate electric contacts on one surface of each thermistor and then breaking individual thermistors from the sheet.
The sheet is scored by an appropriate scoring means for forming a grid of score marks defining a plurality of small size rectangles. Each becomes a thermistor wafer. The scoring means further scores at least one surface of the sheet for removing some of the contact material on each thermistor wafer to define the plurality of contacts on that surface of each thermistor. As a matter of choice, the sheet may be scored for both the wafer forming score marks and the contact forming score marks on one surface of the ware, or the wafer forming score marks and the contact forming score marks may be scored on opposite surfaces. The wafer forming score marks are made deep enough so as to cut all the way through the layer of contact material on that surface and to cut partially through the thermistor material itself. Thereafter, when a bending moment of force is applied to the sheet in an attempt to bend it at one of the wafer forming score marks, the sheet snaps at that mark. In this manner, all of the wafers are eventually broken free from the sheet. The contact forming score marks in the contact material are not so deep as the wafer forming score marks, but are sufficiently deep to remove all of the contact material along that mark.
For defining all of the score marks in the sheet of thermistors, various known scoring means may be used. Conventional rubbing or abrading means may be used. It has been found, however, that for the small size thermistors being formed here, this is not the best technique. Another known technique is to blow an abrasive powder under pressure against the surface of the sheet of thermistors. Motion of the stream of powder across the sheet will form the score marks. Yet another known technique is to direct an appropriately intense laser beam at the surface of the sheet of thermistors and to move the beam over the surface for defining the score marks. The intensity of the laser beam and the speed at which it moves over the surface of the sheet determines the amount of material that is burned away and determines the depth and width of the score marks.
The bending moment of force applied to adjacent thermistors to break them apart is applied to the entire body of each thermistor then being separated. Since the contacts on the surface of the thermistor are also separated from each other by a score mark across the thermistor, the potential exists for the thermistor to break where it has been weakened along the score mark separating the contacts.
With thermistors having length and width dimensions that are fairly near to one another, there has not been a significant problem of breakage of the thermistor along the contact forming score mark. In a clinical thermometer of the type described in U.S. Pat. No. 4,317,367, the length and width dimensions of an individual thin thermistor wafer have been relatively similar. When force has been applied on the sheet of thermistors for separating adjacent thermistors, the sheet has reliably broken along a score mark separating the adjacent thermistors.
Wafer-type thermistors may be used in other applications besides a clinical thermometer. For instance, the thermistor may be inserted into a catheter, such as a catheter that may be inserted into a blood vessel. Since the catheter is quite narrow, the length and width dimensions of that thermistor may radically differ from those dimensions of a thermistor that can be used in a clinical thermometer. For a thermistor to have a particular resistance and a particular resistance change in response to a temperature change, the thermistor must have a sufficient mass of thermistor ceramic material. Yet, the thermistor that is used in a catheter, for example a catheter that is inserted into a blood vessel, must be relatively thin in two of its dimensions, its width and thickness dimensions, so that it can be fitted on or inside a catheter to be inserted in a blood vessel. To give the thermistor the needed mass, its length dimension is proportionately greatly increased. The thermistor is installed in the catheter with its length dimension extending along the length dimension of the catheter, so that the resulting catheter is thin in its thickness or diameter. That elongate thermistor still has a score mark through its contact material for defining two electric contacts on one surface of the thermistor. For convenience in electrically connecting the thermistor in the resistance measuring circuit (which is calibrated to measure temperature), the score mark in the contact material on one surface for defining the two contacts of the thermistor extends along the short, width dimension of the thermistor. Because the thermistor is quite long in its length dimension, there is a long lever arm between the contact forming score mark in the thermistor on the one hand, and the opposite, short length edges of the thermistor wafer that are spaced away from this score mark. When it is desired to separate the adjacent thermistors at those short length edges, and a bending moment of force is applied to attempt to snap the thermistor sheet at those wafer forming score marks at the short length edges, the moment of force applied to an individual thermistor may also be large enough that, on some occasions, an individual thermistor will itself snap apart and break at the contact forming score mark, which destroys the thermistor. This tendency is enhanced because the contact forming score mark has heretofore been a straight line, parallel to two of the edges of the thermistor, and the bending force applied to separate adjacent thermistors along those two edges also tends to snap the thermistor on the contact forming score mark.